Fuel consumption conversion

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pennycar9

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I have a NH220 Cummins fitted to the boat and have,up till now, absolutly no idea what the fuel consumption is.
Today however I have found a performance graph which shows that the consumption is .38lbs per HP per hour, which equates at 150hp at 1400 rpm therefore the consumption is 57Lbs an hour. The question is what is 57lbs per hour in Ltrs/ hour or Gallons /hr?
 
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I have a NH220 Cummins fitted to the boat and have,up till now, absolutly no idea what the fuel consumption is.
Today however I have found a performance graph which shows that the consumption is .38lbs per HP per hour, which equates at 150hp at 1400 rpm therefore the consumption is 57Lbs an hour. The question is what is 57lbs per hour in Ltrs/ hour or Gallons /hr?

[/ QUOTE ]It is vital to understand any units you use. The figure quoted by Cummings will be mass per horsepower-hour, not mass per horsepower per hour. There is a huge difference. A horsepower-hour is a unit of energy (like a gallon of paraffin, a kWh, or a ton of coal or a Therm of gas). A horsepower per hour is virtually meaningless.

We get the same problem with batteries when people talk about 'amps per hour' when they mean 'Ampere-hours'.

So, why am I being so pedantic? It is because if people look carefully at the units the actually make sense, and they can work with them. The maths is very simple and so is the physics. But when people talk about 'amps per hour' or 'horsepower per hour' it is impossible for anyone to know what they are talking about and impossible for them to work it out for themselves.

In metric, most engines are specified in grammes of fuel per horsepower-hour. Older engines will be specified in the old Imperial units which need to be converted but the concept is identical. Let's work in metric....

Most modern yacht diesel engines have a "specific fuel consumption" of around 180g diesel per horsepower-hour. What does this mean? We all understand what 180g of diesel means (but it must be corrected by the specific gravity to convert to litres). What is a 'horsepower-hour'? (A horsepower is about 760 Watts).

A horsepower-hour is the amount of work (done) -- i.e. an amount of energy delivered -- in a defined time. If you ran a one hp motor loaded to one hp for one hour then the work done (by the shaft) would be one horsepower-hour. Or around 0.760 kilowatt-hours (by convention written as kWh). If you are paying 10p per kWh then running that load for an hour will cost you the 7.6p that went into the load, plus any losses in the motor -- around 10p. Your horse would have needed some oats for fuel and your yacht engine would have needed diesel -- around 180g of diesel.

For my yacht, I have taken the Yanmar figures for my engine, from the curves, and recorded in the log book the fuel consumption vs hp vs RPM across the operating range.....

Sorry, the table hasn't tabulated but should be readable...I haven't sorted out how to tabulate it and need to get on....

hp litres per hour RPM Speed kts (liveaboard)
10 1.5 1600
20 3.1 1900 5.5
30 4.6 2200
40 6.0 2400
50 7.7 2600 7.0
60 9.2 2725
70 10.7 2900
80 12.2 3025
90 13.8 3125
100 15.3 3250 8.0
 
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I have found a performance graph which shows that the consumption is .38lbs per HP per hour, which equates at 150hp at 1400 rpm therefore the consumption is 57Lbs an hour.?

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These are test bed figures measured against a dynamometer load. They will only apply if your propellor is absorbing 150 H.P. at 1400 engine rpm, a difficult figure to establish. If the absorbed power is different, your fuel consumption calculation will be in error. The best way of measuring is over a known distance and time.
 
I don't see the difference.

Saying that an engine is giving 150hp at 1400RPM does not make sense to me. Does this mean that load has no effect on the consumption?

If I run my engine at 2200RPM with no load the consumption is minimal. If, however, I am tied up to the quay and running a basin trial, I can still be running at 2200RPM - but there will be one hell of a difference in the quantity of fuel that is consumed.

If my chart says that consumption at 'x' RPM is '250 grammes per kilowatt per hour' then, if I am only taking 1 kilowatt I will be burning 250 grammes for every hour that the engine is running; whereas if I am taking 12 kilowatts out of the engine, the consumption will be 250 x 12 = 3000 grammes each hour.

This is what is used to decide at what engine speed to proceed, given the prevailing seastate.
 
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I don't see the difference.

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You mean that you cannot see the difference between hp-hr and 'hp per hour'? Or the difference between Ah and 'amperes per hour'? That is a very common misunderstanding on these forums. What we need is a competent post explaining the difference, written by someone with teaching skills. A science, teacher, perhaps.

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Saying that an engine is giving 150hp at 1400RPM does not make sense to me. Does this mean that load has no effect on the consumption?

[/ QUOTE ]. No. He said that the specific fuel consumption of that figure was obtained from a graph at the point at which the power and speed were as quoted.

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If I run my engine at 2200RPM with no load the consumption is minimal. If, however, I am tied up to the quay and running a basin trial, I can still be running at 2200RPM - but there will be one hell of a difference in the quantity of fuel that is consumed.

[/ QUOTE ] These curves are produced for marine engines driving typical marine gearboxes and propellors. You use them to determine the fuel consumption under load.

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If my chart says that consumption at 'x' RPM is '250 grammes per kilowatt per hour' then, if I am only taking 1 kilowatt I will be burning 250 grammes for every hour that the engine is running; whereas if I am taking 12 kilowatts out of the engine, the consumption will be 250 x 12 = 3000 grammes each hour.

[/ QUOTE ] Specific fuel consumption is given in hp-hr or kWh, not kW per hour. To use these charts you firstly decide what RPM you are looking at. Suppose you normally cruise at 1900 RPM then check what power is being developed at that RPM assuming you have the appropriate gearbox and prop. You will get a figure of kW or hp. Being in neutral, dirty props, towing vessels, or rough seas will invalidate the curve. Now you need to check the specific fuel consumption (maybe a curve of sfc vs power?) for that power output. The sfc is fairly flat from 10% to 90% load. The amount of fuel you will use in one hour is given by the specific fuel consumption in hp-hr (or kWh) times the hp (or kW). That gives you the mass of fuel used in one hour. To get gallons or litres you multiply by the density (wiki). If you know your fair weather boat speed vs RPM you can now construct a table showing your endurance and range vs different speeds (RPMs). I keep mine in the front of my logbook so if ever I needed to make a decision to divert I could decide in minutes whether to go for range or endurance, and what my options are.
 
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Can I ask what size Yanmar is fitted to your boat?

[/ QUOTE ]Hi Shorn, I have a Yanmar 4LH-TE turbocharged 110hp.
 
Charles -- well done, the only person who actually answered the Q -- I had meant to do the sums and include it in my answer but forgot /forums/images/graemlins/blush.gif
 
Thank you all for your comments. My head hurts with the thought of all those sums. It seems that I am looking at a consumption figure of 29-30 lts an hour which gives me a range of about 450 miles.

Once again thank you all for your imput.
Paul Carter
 
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It seems that I am looking at a consumption figure of 29-30 lts an hour

[/ QUOTE ]That seems very high. At the standard sfc the Yanmar would give 150hp at 22-23 litres per hour.
 
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If you ignore mine, that is!

[/ QUOTE ]I'd never ignore you /forums/images/graemlins/smile.gif
 
To produce a meaningful table for fuel consumption v. engine rpm for a particular boat installation requires a propellor power curve. This is a completely different shape from the engine power curve. The two will normally cross near max. rpm on a well matched installation. Calculating fuel consumption using the engine power v rpm graph is not valid for intermediate speeds.
 
That is probably so, but my engine is 35 years old and I would expect it to consume more than the younger equivelents. The old girl still purrs along quite happily though.
 
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That is probably so, but my engine is 35 years old and I would expect it to consume more than the younger equivelents.

[/ QUOTE ]Not a lot has happened to diesels in the last 35 years in the context of specific fuel consumption. Why would you expect it to consume so much more fuel? Where is the unburnt fuel going? Smoke? System losses? It sounds a bit excessive to me..... OK, I acknowledge that you have the answer you wanted but if anyone else has anything to add, to discuss, about this rather large difference in efficiency, it would be interesting.
 
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